Reference signals, such as pilots, may be inserted in a transmitted data stream to assist a receiving entity with various functions, including not only channel estimation but also timing and frequency offset acquisition. A reference signal typically includes one or more modulation symbols known to both the transmitting entity and the receiving entity that are transmitted in a known manner. Since reference signals represent overhead in a system, it is desirable to minimize the amount of system resources used to transmit reference signals (e.g., pilots).
Conventional systems employ various types of reference signals, with varying fixed structures, to provide sufficient measurements and estimations for adaptive multi-antenna operation. For example, a common reference signal is a signal used by many if not all transmitters in a network to facilitate channel estimation. The common reference signal can employ a fixed pilot structure that provides an adequate number and distribution of pilot symbols for most receiving entities under most channel conditions. However, this approach results in a common overhead for all receiving entities. The common overhead becomes difficult to scale up to large numbers of transmit ports (e.g., in massive multiple-input, multiple-output (MIMO)) as well as results in a dense pilot structure that can cause pilot pollution in partially loaded or unloaded cells.
Another type of known reference signal is a channel state information reference signal (CSI-RS) that employs a fixed pilot structure that is significantly sparser than that used for the common reference signal. The CSI-RS is useful for estimating channel quality in frequencies other than those assigned to specific user equipment (UEs) in a given cell. Although CSI-RS results in a smaller overhead, the spacing can be too large in the time domain to train a frequency tracking loop. The CSI-RS may also result in an aliased channel energy response under a long delay spread channel. Thus, there is a need for techniques to provide reference signal spacing and structures that provide more information to estimate to channel conditions.